1. Field of the Invention
This invention relates to a vibration noise filtering system for use in airborne radar systems and more particularly to a system comprised of both a passive mechanical isolator and an electronic active circuit for filtering vibration induced noise in the radar master oscillator signal over a wide frequency spectrum.
2. Description of the Prior Art
Recently, there has been a trend towards employing multi-mode radar systems in today's aircraft utilizing both coherent air-to-ground (mapping) and coherent air-to-air operation modes. Mechanically sensitive quartz crystals are primarily used as the source of the transmittal carrier signal and the receiver mixing signal for most radar master oscillator, RMO, systems. Generally, signal content modulation frequencies of air-to-ground mode operation are in the range from 0.01 Hz to 100 Hz and for air-to-air modes from 100 Hz to approximately 2 MHz. A problem of paramount importance to airborne radar systems is the large vibrations generated within the aircraft as a result of the constant maneuvering thereof. These vibrations are effected across the frequency spectrum and may induce a frequency response in the quartz crystal of the RMO within the modulation frequency spectrum of the received signal content which ultimately may appear as part of the radar image producing false indications or incorrect information, as the case may be.
In the case in which both air-to-ground and air-to-air modes are utilized in an aircraft, the RMO crystal must have both excellent low frequency and high frequency phase stability, respectively. The phase spectral stability requirements of an RMO are normally measured by the quantity referred to as phase spectral density which is a measurement, usually in decibels, of the power in the noise sidebands per unit bandwidth with respect to the total radio frequency power per unit bandwidth. Typical phase stability requirements starting at approximately -35 dB at 2 Hz, decreases at -20 dB/decade to about -80 dB at around 200 Hz. Thereafter, the requirement changes abruptly reaching a plateau value of approximately -120 to -140 dB at around 2000 Hz. One may operate on the phase spectral stability requirement of the RMO using a given mathematical relationship to yield a representative envelope of permissible vibration input. This representation is generally compared with the vibration spectrum which is representative of that which is expected to occur in the particular aircraft in question under worst case conditions. In an exemplary case, the expected frequency vibration envelope may exceed the representative permissible vibration spectrum by 3 or 4 dB at the low frequency end at around the 20 Hz range, while the high frequency vibration input at around 2000 Hz may exceed that permissible by about 40 dB.
A classic approach to reducing or eliminating the high frequency vibration problem is to mount the enclosure of the RMO structurally on one or more stages of passive mechanical isolators which separate it from the vibration source. This approach has been successfully used in some aircraft where only air-to-air mode radar operation was facilitated. Unfortunately, this solution is generally not adequate for the aircraft which employ both air-to-air and air-to-ground airborne radar systems. The reason lies in the limitation of the conventional mechanical vibration isolators.
The vibration input to an aircraft, as a result of maneuvering thereby, normally has a fixed amplitude power spectrum over a wide bandwidth. However, the surrounding mechanical supporting structure of the aircraft creates, at times, resonance "peaking" which results in a vibration source spectrum to the RMO enclosure of both amplified narrow band and relatively lower amplitude broad band vibration noise. This same phenomenon occurs when a mechanical isolator is structurally connected between the RMO enclosure and the vibration source for inhibiting the transmissibility of vibration noise to the insulated RMO enclosure. More specifically, the conventional mechanical isolators may result in amplification factors on the order of 8 to 10 dB at their resonance frequency. For example, a 1.0g broad band spectrum vibration source input may result in a 2.5 to 3.0g amplified input to the isolated RMO enclosure in the vicinity of the resonant frequency of the isolator which is normally set at around 20 Hz for air-to-air vibration protection. This might be acceptable for individual air-to-air radar systems, but the amplification vibratory noise at around 20 Hz further complicates the air-to-ground mode radar operation which is anticipated to be 3 to 4 dB out of specification already as described above.
Attempts to solve the problem by conventional means leads to possibly increasing the number of stages of mechanical vibration isolation or lowering the isolation cut-off frequency. Either case will result in size and weight increases and possibly larger sway space requirements which are normally undesirable to aircraft designers. Attractive solutions of unconventional means have been suggested to include an active isolator, either in cascade or parallel with the passive isolator, both disposed between the vibration source and isolated mass for the purposes of damping the "peaking" effects of the passive isolator on the isolated mass at low frequencies. Known active systems typically have used hydraulic damping principles to counteract the movement of the isolated mass at undesirable frequencies.
The U.S. Pat. No. 3,606,233 issued to Scharton et al. on Sept. 20, 1971 and U.S. Pat. No. 3,807,678 issued to Karnopp et al. on Apr. 30, 1974 both teach a hybrid vibration isolation system comprising both a passive and an active isolator wherein the active isolator provides damping of the amplified vibrations produced by the passive isolator at the resonant frequency thereof. The active systems of Scharton et al. and Karnopp et al are basically hydraulic in operation and include a pressurized hydraulic fluid energy source, a hydraulic servo valve coupled to an hydraulically operated piston actuator for converting the hydraulic energy into a controllable form for effecting a counteraction to the undesirable movement of the isolated mass, and an electronic feedback system for monitoring the instant velocity of the isolated mass and governing the servo valve with respect to said movement.
While these active systems do cooperate effectively with the passive isolators to regulate the transmissibility characteristics of the overall system, they also have their disadvantages. In general, isolation systems of this type must be carefully designed, maintained and controlled to avoid unstable behavior. In addition, such hybrid systems require an auxiliary source of hydraulic energy and equipment to convert this energy into a convenient form for use thereby. Accordingly then, a hybrid system having an active device for complementing a mechanical passive isolation system in its resonant frequency range appears attractive for use in preventing vibration induced noise in the RMO of airborne radar employing both air-to-ground and air-to-air modes of operation. A hybrid system which could eliminate the necessity of an auxiliary energy source and conversion equipment and maintain effective vibration transmission regulatory characteristics would be preferred. One in which the active portion could be implemented in solid-state electronics would be even further preferred.